The Mössbauer recoil-free fraction and structure. Part 1. α, β, γ, δ-tetraphenylporphinatodihydroxytin(IV)-bis(chloroform)-bis(carbon tetrachloride) solvate

Abstract

The structure of α, β, γ, δ-tetraphenylporphinatodihydroxytin(IV) (TPPSn(OH) 2) has been determined as its bis(chloroform)-bis(carbon tetrachloride) solvate by Patterson and Fourier techniques to a final ‘R’-factor of 15.14% for 4380 independent reflections. Purple crystals of the material are triclinic, space group P1, with a = 12.229(6), b = 12.310(9), c = 9.592(8) Å, α = 102.58(1), β = 109.64(5), γ = 91.78(8)°, and are composed of stacks of non-interacting TPPSn(OH) 2 molecules, with the solvate molecules occupying spaces in the lattice. The geometry at the tin is only slightly distorted from regular octahedral, with SnN bonds of normal length (2.09(5) Å), but with very short bonds to oxygen (2.00(3) and 2.03(5) Å). Tin-119 Mössbauer resonance area data have been collected in the temperature range 77 ⩽ T ⩽ 153 K, and a semi-logarithmic plot of resonance area versus temperature is linear in this temperature range, yielding values of the logarithmic temperature coefficient of the recoil-free fraction, a, and of the characteristic Mössbauer temperature, θ M, of −0.01193 K −1 and 122.3 K, respectively. The data have also been used to evaluate the isotropic mean-square amplitudes of vibration, <x iso(T) 2>, of the tin atom, normalised to the value at 298 K available from the crystallographic study, in the same temperature range. Root-mean-square values of the vibrational amplitude range from 0.125 Å at 77 K to 0.148 Å at 153 K. From the Goldanskii-Karyagin effect data, it was concluded that the sign of the electric field gradient, V zz, assumed to be coincident with the pseudo-four-fold axis of the molecule, is negative, and that the tin atom vibrates with greater amplitude in the plane of the porphyrin residue (perpendicular to V zz). The temperature coefficient of the root-mean-square amplitude of vibration is significantly larger in the out-of-plane direction (along the OSnO axis) (4.184 × 10 −4 Å K −1 than in the in-plane direction (2.618 × 10 −4 Å K −1), reflecting the constraining nature of the porphyrin residue. Absolute values of the recoil-free fraction, f a were estimated to be 0.100 at 77 K reducing to 0.0404 at 153 K.